Voltage-controlled oscillator (VCO) typically generates a signal having an oscillation frequency determined by a control signal. To save power in electronics devices such as portable devices, VCO is increasingly designed to operate with power supply having a lower voltage level.
In an approach, a VCO including NMOS transistors, PMOS transistors and a current source is provided. However, the VCO requires a large voltage headroom consisting of an NMOS threshold voltage, a PMOS threshold voltage, and a saturation voltage of the current source. As a result, the voltage headroom cannot be reduced when a lower supply voltage is used for the VCO.
Conversely, in another approach, an NMOS or PMOS VCO is provided and is able to have a reduced voltage headroom. However, it has worse performance such as poor phase noise performance.